Speak to Renal Bloodscalp on the second floor of the Shadow Thief Guildhall, which can be found northwest in the area. He will ask you to infiltrate Mae'Var gang of thieves, which is a part of the Shadow Thieves. Renal needs prove that Mae'Var is plotting against him. Do whatever task is required to earn his trust. He will give you a Letter of Transfer.
Enter Mae'Var's Guildhall and show the letter to Gorch behind the counter. He will grant access to the guildhall. Go through the door and then down the stairs beside you. Behind a secret door, you will find Mae'Var torturing a thief named Lin. He will give you your first assignment, depending on your Reputation.
If your reputation is 10 or higher, you will be given the task to steal from the Temple of Talos. If your reputation is 9 or lower, you will have to steal from the Temple of Lathander. Make sure it is nightfall before you do either of the two tasks.
Enter the Temple of Talos in the Athkatla Temple. Pick the lock of the chest in Weathermistriss Ada's chamber and take the Necklace of Talos. Give the necklace to Mae'Var in his Guildhall and he will tell you to report to Edwin, his second man in command.
Go to the Temple District. The entrance to the Temple of Lathander is north in this area. Enter the Temple of Lathander. The Statue of Lathander is in a strongbox located in the temple. However, it is guarded by Dawnmaster Kreel is hanging around. At day there are also two Morning Knights. You can't burgle the temple's strongbox so long as anybody is there. If you do, all NPC's in the temple will turn hostile towards you.
Have your party Thief hide in the shadows, and have everybody else leave the area. Wait for Dawnmaster Kreel to go away. When he is out of sight, pick the lock on the strongbox and take the Statue of Lathander. Go back to Mae'Var and deliver the statue and he will tell you to report to Edwin, his second man in command.
Get to the third floor of the guildhall and speak to Edwin, a former companion in Baldur's Gate. He will require that you kill a Cowled Wizard named Rayic Gethras for him, who lives in the docks district. Enter Rayic Gethras' house to the west and defeat the wizard on the third floor. Return to Edwin to report your success. Next he will ask you to retrieve some documents from a merchant named Marcus.
Enter the Sea Bounty Tavern just east of the guildhall. Talk to Marcus, you can either pay him or threaten him to hand them over, if your strength is high enough. Or you can try to pickpocket the documents from him without even needing to speak to him, though you need a high enough pickpocketing skill. If you're a female with high charisma he may just give them to you with some flattery. Give the documents to Edwin and he will tell you to report to Mae'Var for your final assignment.
Return to the cellars of Mae'Var's Guildhall and speak with Mae'Var. He will require that you kill a traitor named Embarl. Enter the second floor of the Sea Bounty and speak to Embarl. He overheard Mae'Var say that he wants to kill Renal Bloodscalp but he has no proof. Ask him to give you his dagger and let him leave.
- Give the dagger to Mae'Var as proof that you killed Embarl.
- Speak to Edwin once more and he will reveal that he wishes to betray Mae'Var. He will give you Mae'Var's Strongbox Key and he will offer to join your party.
- Go down a floor and use the key to open the stronbox in the north chamber. Take Mae'Var's Letter from the box.
Give the letter to Renal Bloodscalp in the Shadow Thief Guildhall. He will thank you for proving Mae'Var's guilt and will ask you to return to Mae'Var and kill him. Return once again to Mae'Var's Guildhall and fight your way through to Mae'var in the cellar. Once you have killed him, return to Renal Bloodscalp for a handsome reward.
If you are a thief, he will offer you Mae'Var's former position as head of the thief guild.